In general, in an image forming apparatus capable of copying onto a large-sized sheet, such as A0, A1, etc., if such a large-sized sheet is stored as it is or if the large-sized sheet is handled as it is, the storage space required is large, or handling is inconvenient, and as such, normally, the sheet is folded for storing and handling.
However, if a sheet folding operation is performed manually, a significant amount of time is needed, so that a time required for folding a sheet may end up being longer than a time required for copying.
In light of the above, there is a system in which is arranged, in a sheet discharge path of an image forming apparatus, a sheet folding apparatus which can fold a sheet automatically, making it possible to automatically fold in succession large-sized sheets copied, as is discussed generally in JP 5-238635, JP 11-349218, JP 2004-67266, and JP 2006-335500.
In a sheet folding apparatus for use in such a system, there may be included a unit (an off-line operation) for folding a sheet which was inadvertently copied and discharged without folding in advance, or a large-sized sheet which was produced by a different image forming apparatus, as well as (an on-line operation of) automatically folding a sheet discharged from an image forming apparatus.
When paper folding is actually performed by such a system, for a standard-sized sheet such as A0, A1, etc., paper folding can be performed using a predetermined size for each face of a sheet to be folded. In an image forming apparatus capable of copying onto the large-sized sheet, copying may be performed onto a long sheet such that a length in a sub-scanning direction amounts to several meters, so that it is required to perform paper folding even for such a non-standard long sheet.
A paper folding apparatus disclosed in JP 5-238635 automatically adjusts a folded length of each folded portion to fold even a sheet other than one having a standard length to a specified dimension.
JP 11-349218 discloses a paper folding method and a paper folding mechanism that orderly form a folding edge in a desired folding mode.
JP 2004-67266 discloses a paper folding machine which ensures that unnecessary folds are not produced when a folded sheet is discharged.
JP 2006-335500 discloses a paper folding apparatus which reduces unevenness in a folding dimension even for a large-sized sheet having a curled tip.
FIG. 23 is a schematic diagram showing an operation of a related-art sheet folding apparatus. In FIG. 23, the sheet folding apparatus 1 includes a conveying roller pair 3; a first folding roller pair 6 and a second folding roller pair 7 that are arranged in an opposing manner to the left and the right of a lower portion thereof; a first sheet guiding member 4 and a second sheet guiding member 5 that selectively guide a sheet P conveyed from the conveying roller pair 3 in each folding roller pair direction; and a lower sheet face guide 10 which guides a lower face of the sheet P. The sheet P is fed by the conveying roller pair 3 and conveyed, and selectively guided to the first folding roller pair 6 or the second folding roller pair 7 by an operation of the first sheet guiding member 4 or the second sheet guiding member 5. The guided sheet P is folded a predetermined number of times by being fed between the first folding roller pair 6 or the second folding roller pair 7 while sliding on an upper face of the lower sheet face guide 10. This is accomplished by an operation of the first sheet guiding member 4 or the second sheet guiding member 5 in alignment with repetition of forward and reverse rotations of the first folding roller pair 6 and the second folding roller pair 7.
A first sensor 8 is arranged downstream of the first folding roller pair 6 in the sheet conveyance direction. The first sensor 8 senses the front end of the sheet folded by the first folding roller pair 6. A second sensor 9 is arranged downstream of the second folding roller pair 7 in the sheet conveyance direction. The second sensor 9 senses the front end of the sheet folded by the second folding roller pair 7. In FIG. 23, a number 2 denotes an inlet sensor.
Another known folding method is one in which crest folding is repeated. A folding finishing state shown in FIG. 24A is a state in which an A0-sized sheet is folded, resulting in basic folding. In this case, crest folding is performed twice with a title column facing up. Moreover, in a similar manner, with respect to A0-sized bag folding, crest folding is performed twice with the title column facing up. This folding, which cannot be handled by the related-art sheet folding apparatus, is handled by performing crest folding and trough folding from the title column side as shown in FIG. 24B.
There are other folds which require finishing in which there are successive folds in the same direction. These are folds which are often used in notices, brochures, etc., including, for example, an inner three fold as shown in FIG. 24C and a gate fold as shown in FIG. 24D. A Z-fold shown in FIG. 24B is achieved by folding to successively perform trough folding or to successively perform crest folding.
However, the sheet folding apparatus 1 often causes displacement of the folded sheet shown in FIG. 23C when the apparatus transfers the folded sheet between rollers.
FIG. 23A shows a state in which a sheet is folded at first, the front end Pt of the folded sheet is guided to a conveying roller pair 3 by reversing the second folding roller pair 7 and with a second sheet guiding member 5. In this state, a gap occurs between an aspect from the front end Pt to the folded part Pb of the folded sheet and another aspect of the sheet part after the folded part Pb.
The diagram shown in FIG. 23B is a state in which the front end Pt of the folded sheet is inserted in the conveying roller pair 3 again in the state with the gap shown in FIG. 23A.
The diagram shown in FIG. 23C is a state in which the folded part Pb of the folded sheet is guided from the second folding roller pair 7 to the first folding roller pair 6. When the folded part Pb of the folded sheet is nipped and conveyed by the first folding roller pair 6 in this state. The folded part Pb of the folded sheet is formed to the displacement of the folded sheet.